2i6 American Quarterly Microscopical Journal. 



of an iron bed, 60 inches long and 14 inches wide. V-shaped 

 grooves, 6 inches apart, run the entire length. In the center 

 of the bed, a fine-toothed rack reaches from end to end. Two 

 sliding plates are carried along the ways by means of a pinion 

 set in the center of the plates, and working so loosely in the 

 rack that the slides are free to follow the law of gravity. A 

 microscope is attached to each plate, giving the form usually 

 adopted. Instead of two microscopes, however, it is found 

 better to use but one. The microscope plate is followed on 

 either side by plates terminating in tempered steel stops, 

 which are at will either made free or clamped firmly to the 

 bed of the comparator. If one wishes to compare two meters, 

 the method of proceeding is as follows: 



{a) One stop is set at or near one end of the bed. 



{b) The meter with which comparison is to be made is placed 

 in position under the microscope, so that contact is made be- 

 tween the end line and the zero line of the eye-piece micrometer. 



[c] The microscope plate is then moved by means of the 

 rack and pinion till the other end line forms contact with the 

 zero line of the micrometer. 



(d) The second stop is then brought up against the other 

 end of the plate and adjusted so that when contact takes place 

 between the stops, contact also takes place between the end 

 line and the zero line of the micrometer. 



{e) Having made the adjustment of the stops perfect, the 

 meter to be compared is then placed in position. When contact 

 is made with the first stop, by mechanical adjustment, the end 

 line is brought in contact with the zero line of the micrometer. 

 The microscope plate is then brought into contact with the 

 second stop. If the other end line is now in coincidence with 

 the zero line of the micrometer, the two meters have the same 

 length. By noting the number of divisions which the end line 

 falls short of, or passes beyond the zero line of the micrometer, 

 the difference in the entire length can be found ; the only 

 element yet unknown being the value of one division of the 

 micrometer. 



After the comparison has been made, it is better, or a matter 

 of precaution, to again compare the standard with the distance 

 between the stops. Since the stops can be set in actual con- 

 tact with the microscope plate at either end, it is obvious that 

 this method admits of a comparison of short spaces as well as 



